The objective of my proposal is to fully characterize antibody molecules belonging to the class designated as IgM, in order to lead to a better understanding of their biological functions in various normal and pathological conditions. IgM paraproteins, which are believed to be a product of a unique clone that underwent malignant transformation, are generally regarded as homogeneous proteins because of their high homogeneity in free electrophoresis. However, some of the data available in the literature as well as results of my own preliminary experiments clearly suggested that at least L-chains from IgM proteins are heterogeneous. Therefore this proposal is made to clarify the nature of the heterogeneity of the constituent polypeptide chains of monoclonal IgM protein. Particular emphasis will be placed to correlate the results of such studies to covalent structure of IgM protein.